1. Field of the Invention
The present invention relates to radar detection circuits and more particularly to baseband pulse detection circuits for expanded time electromagnetic ranging systems. The invention can be used to accurately detect the time of occurrence of pulses for impulse and pulsed radar, Time Domain Reflectometers (TDR), pulsed laser sensors and radiolocation systems.
2. Description of Related Art
Short range, high resolution pulse-echo ranging systems, such as impulse radar, TDR and pulsed laser rangefinders often transmit a sub-nanosecond wide pulse. Guided wave radars (GWR), also known as “electronic dipsticks” since they employ a single-wire electromagnetic guide wire, also often transmit a sub-nanosecond wide pulse and can be found in, for example, industrial pulse-echo TDR systems used to measure liquid levels in tanks. These systems usually operate in an expanded time mode, whereby a transmit pulse rate is slightly higher than a receive gate frequency, or sampling rate, to produce a stroboscopic effect in the form of a down-sampled, expanded-time signal.
The stroboscopic effect produces detected output pulses that resemble realtime sub-nanosecond pulses, but they occur on a vastly expanded time scale. Time expansion factors of 100,000 to 1-million are common. Accordingly, a 1-nanosecond wide realtime transmit pulse can produce a sampled output replica pulse having a 1-millisecond expanded time duration. At 1 ms duration, pulse detection and other processing is vastly easier. Examples of expanded time GWR architectures are disclosed in U.S. Pat. No. 5,609,059, “Electronic Multi-Purpose Material Level Sensor,” by the present inventor, Thomas E. McEwan, and in U.S. Pat. No. 6,452,467, “Material Level Sensor Having a Wire-Horn Launcher,” also by the present inventor. An example of an expanded time laser ranging system is disclosed in U.S. Pat. No. 5,767,953, “Light Beam Range Finder,” by the present inventor. An example of an expanded time radar is disclosed in U.S. Pat. No. 6,137,438, “Precision Short-Range Pulse-Echo Systems with Automatic Pulse Detectors,” to the present inventor.
High accuracy range determination depends on precisely detecting a time duration between a transmit pulse and a receive pulse. However, the transmit and receive pulses are often coupled to a receiver through different networks and thus may have different waveshapes. This makes precise range measurement extremely difficult, if not impossible. For example, a transmit pulse may be coupled to a receiver through a distortion-free coupler, while receive pulses may travel through, for example, an antenna, which can differentiate a pulse multiple times. Consequently, the transmit pulse waveform may consist of a single lobe of a half sinewave while the receive pulse waveform may have degenerated into several alternating polarity lobes. For high ranging precision, it is beneficial to detect the same point on transmit and receive pulse waveforms that have the same waveshape as they issue on a common line from the receiver. Preferably, this point is also independent of pulse amplitude variations, i.e., a zero axis crossing point.
One prior approach to the detection problem is a fixed threshold detector that triggers on the first pulse lobe to cross a threshold. Unfortunately, variations in received signal amplitude and pulse shape make this approach unattractive. In order to maintain 1-picosecond detection accuracy on an pulse having a 100 ps risetime, the detection point would need to be consistent to 1% of the pulse amplitude. Receive signals rarely have such consistency.
Another prior approach is time-of-peak (TOP) detection. U.S. Pat. No. 5,457,990, “Method and Apparatus for Determining a Fluid Level in the Vicinity of a Transmission Line,” by Oswald et al, discloses the use of a threshold detector combined with a TOP detector. When a pulse exceeds a threshold, a TOP detector is enabled and the pulse peak is detected by differentiating the pulse and then detecting the zero-axis crossing of the derivative to find the exact time-of-peak. This approach, as disclosed in the '990 patent, has serious limitations. First, the transmit pulse has a substantially different shape, a monocycle shape, than the receive pulse, which has a “W” shape. Consistent, precision time-interval detection is difficult if not impossible between two different pulse shapes. Second, TOP detection itself has inherent limitations: (1) the peak region of a pulse has the slowest voltage rate of change and is therefore the most susceptible region on the pulse to noise, and (2) the peak region is the least accurate for range timing since it is nearly flat and a small voltage error can result in a large timing error upon detection.
Another prior approach has been disclosed in co-pending U.S. Patent Application Ser. No. 11/355,845, “Carrier Phase Detection System for Radar Sensors,” filed on Feb. 16, 2006 by the present inventor filed on Feb. 16, 2006, now U.S. patent 7,379,016. This system operates by detecting the TOP of an expanded time RF burst envelope within an analysis window of time and then using that detection event to gate a carrier phase detector. The carrier phase detector detects the zero axis crossings of each sinewave cycle within a burst. A zero axis crossing of a selected cycle is gated by the TOP detection. Therefore, the accuracy of the detection is directly tied to the selected sinewave zero axis crossing, which is highly accurate, and not to the TOP detection accuracy. Limitations to this approach include: (1) the requirement for an envelope detector to detect an envelope of a plurality of detected cycles, i.e., a multi-cycle sinusoidal burst, and (2) changes in envelope shape due to target characteristics can produce jumps to another cycle within the burst, resulting in large errors.
Prior pulse detection approaches present hurdles to ranging precision on the order of 1 ps, particularly when transmitting and receiving mono-lobe or monocycle pulses having a duration of, for example 0.1 to 1 ns. Thus, a new pulse detector is needed.